(Not applicable).
(Not applicable).
(1) Field of the Invention
The present invention relates generally to a reciprocating compressor utilized to pump fluids, and more particularly to an improved unloader for varying the internal volume of a reciprocating compressor cylinder.
(2) Background Information
Natural gas is distributed through pipelines throughout the country. A series of pumping stations are located at predetermined intervals along the pipelines to move the gas throughout the system. Each pumping station includes one or more compressors with a plurality of reciprocating cylinders for moving the gas through the pipeline.
An unloader is a device which will selectively increase or decrease internal volume/clearance of a reciprocating compressor cylinder in order to increase or decrease the unit load and accommodate changing suction and discharge pressures. It is beneficial to maintain a compressor engine loaded near its rated horsepower to obtain the lowest specific fuel consumption rate and the highest compressor throughout. This is accomplished by constantly varying the unit load depending upon various operating conditions. Such conditions include: (1) the type/quality of gas being compressed; (2) operating pressures and temperatures; (3) compressor speed; (4) compressor cylinder size (diameter of the bore and length of the stroke); and (5) fixed and variable clearance volumes.
Single stage unloaders are currently used to permit the clearance volumes in the compressor cylinders to be varied, to thereby attempt to keep the engine loaded near its rated horsepower. However, single state unloaders still suffer several drawbacks.
First, there are a limited number of locations around a compressor cylinder for installation of an unloader. Thus, the amount of clearance volume available to a cylinder is also limited.
In addition, if the single stage unloader provides a large volume change, the large volume change may lead to cylinder single acting operation where gas is compressed and expanded without the gas flowing through the cylinder. This operation leads to high temperatures that may exceed material limits.
It is therefore a general object of the present invention to provide an improved multistage unloader for reciprocating compressor cylinders.
Another object of the present invention is to provide improved unloader operation, with multiple volumes in place of a single volume, which permits the selection of smaller clearance volumes, allowing a compressor to maintain high volumes while keeping the engine loaded near its rated horsepower.
A further object is to provide an improved multi-stage unloader which permits a greater selection of internal clearances in the same space as a single stage unloader.
These and other objects of the present invention will be apparent to those skilled in the art.
The unloader of the present invention includes a first volume bottle with a cavity formed therein of predetermined volume, and a port in the forward end with a plug operably mounted to selectively open and close the port. A second volume bottle is stacked on the first volume bottle and has a port in its forward end with a movable plug to open and close the second bottle port. The second bottle is arranged such that the second port is connected to the gas stream of the first and second bottles. Actuators are connected to the plugs in the first and second bottles to selectively open only the first bottle or both bottles together, to thereby provide a two stage internal clearance for the reciprocating compressor cylinder.